The Teletext system is an information broadcasting system in which Pages of text and graphical symbols are transmitted in coded form on otherwise unused television lines during the field interval. The decoded Pages are intended to be displayed in place of, or added to, the television picture. Newsflashes and Subtitles may be inset in the picture. According to the Teletext system as presented by the BBC's CEEFAX or IBA's ORACLE service, one of the otherwise unused lines of the television field-blanking interval is used to carry information for one Teletext Character Row. Such a line is referred to as Data-line.
At least one of the seven colors, white, yellow, cyan, green, magenta, red and blue, is used to depict the display character in the character rectangle. The background color which is filling the parts of the character rectangle not occupied by the character itself may be blank or any one of the seven display colors.
In displaying the pages, the operator may select a combination of colors for the display character and the background. Since the colors are established through a so-called additive color mixing system, the total driving current for every color is not the same. Therefore, in some color combinations, the brightness of the background results much brighter than the same of the display character, and vice versa, and, in such cases, it is difficult to distinguish the display character over the background.
Table 1 shows a driving current distribution for each of the seven colors.
TABLE 1 ______________________________________ Driving Current Color Red Green Blue Total on the driving driving driving driving screen current current current current ______________________________________ White 2 2 2 6 Red 2 2 Green 2 2 Blue 2 2 Yellow 2 2 4 Magenta 2 2 4 Cyan 2 2 4 ______________________________________
In Table 1, the values in each row do not present the actual operating value, but merely shows the ratio of three driving currents. For example, white on the screen is established when the red driving current, green driving current and blue driving current are combined in a ratio of 2:2:2. Therefore, the total driving current for white amounts to 6. In another example, red on the screen is established when the red, green and blue driving currents are combined in a ratio of 2:0:0. Therefore, the total driving current for red amounts to 2. In the above examples, it would be quite difficult to distinguish red characters from the white background.